1. Field of the Invention
The invention relates to electric toothbrushes.
2. Description of Prior Art
Many toothbrushes are known and widely used having a set of bristles mounted to a brush head that is driven by an electric motor inside a toothbrush handle. The motor may be powered by a battery, also inside the handle, or from a power supply socket adjacent a point-of-use. As such, the brush head can be rotated and/or vibrated by the motor to enhance the operation of the toothbrush for cleaning teeth. For vibrating the brush head, it is already known to provide an eccentrically mounted weight inside the handle directly coupled to the motor. As a result the handle is vibrated and this vibration is transmitted to the brush head in use via a shank of the toothbrush. This means that the user""s hand is vibrated. Inherently the vibrations are dampened by the user""s grip. When using the toothbrush, the handle vibrations are not comfortable for the user and waste, in effect, considerable mechanical energy that must be supplied by the motor.
It is an object of the invention to overcome or to at least reduce this problem.
According to the invention there is provided an electric toothbrush having an elongate handle, an electric motor inside the handle, a drive shaft rotatable about a longitudinal axis of the toothbrush inside an elongate shank, a brush head mounted at a remote end of the shank carrying bristles extending general transversely to the longitudinal axis from the brush head, an eccentrically unbalanced drive shaft extension that is constrained by the brush head to be freely rotatable within a cavity in the brush head about said longitudinal axis, in which the brush head and the shaft extension are flexibly coupled to the remote end of the shank and to the drive shaft, respectively, such that when the drive shaft is rotated the brush head is caused to vibrate relative to the shank.
The drive shaft extension may be constrained by at least one bearing in the brush head that is positioned about said longitudinal axis.
The shaft extension may comprises a shaft rotatable about the longitudinal axis and a sleeve that fits over the shaft having an off-centre channel in which the extension shaft fits. The sleeve is preferably made of metal.
A resilient bush may be arranged to hold and connect the brush head to the remote end of the shank. The brush may be formed by over-molding resilient material over and form connection between the brush head and the remote end of the shank.
A resilient tube may be used to drivingly connect the shaft extension to the drive shaft.
Preferably, the drive shaft is driven at between 10000 and 12000 revolutions pre minute.